This invention relates to a light emitting device, in particular, combining a light emitting element like a semiconductor light emitting element and a wavelength converting means like a fluorescent element.
Light emitting devices combining LEDs (light emitting diodes) or other semiconductor light emitting elements and fluorescent elements have been remarked as inexpensive, long-lived light emitting devices, and their development is being progressed. Light emitting devices of this type have the advantage of providing emission colors conventional semiconductor light emitting elements could not realize.
Usually, semiconductor light emitting elements emit light upon re-combination of carriers injected into their active layers, and emission wavelengths are determined by energy band gaps of the active layers. For example, monochromatic emission has been realized, such as red and yellow with semiconductor light emitting elements using InGaAlP compounds, and green and blue with those using InGaN compounds.
However, to realize a certain mixed color by using those conventional semiconductor light emitting elements, it has been necessary to combine some light emitting elements for different colors and control optical outputs of individual light emitting elements by adjusting their current values. Therefore, the device configuration was inevitably complicated and needed troublesome adjustment.
In contrast, light emitting devices configured to emit light by wavelength-converting light emitted from semiconductor light emitting elements by means of fluorescent elements are advantageous in realizing a color heretofore impossible with a single semiconductor light emitting element by changing fluorescent elements or their combination.
A white light emitting device, described in “Compound Semiconductor” Vol. 5, No. 4, 00.28-31, is one of light emitting devices combining semiconductor light emitting elements and fluorescent elements. This light emitting device realizes white emission by mixture of two colors from a semiconductor light emitting element for blue light and a YAG:Ce fluorescent element excited by that blue light to emit yellow light.
FIG. 16 is a cross-sectional view illustrating a rough configuration of a conventional light emitting device of this type. A semiconductor light emitting element 802 is placed in an opening 801 formed in a package (resin stem) 800, and a sealing resin 804 is buried to encapsulate the semiconductor light emitting element 802. The resin 804 contains a fluorescent element 810.
The resin stem 800 has leads 805, 806 shaped from a lead frame, and a resin portion 803 molded to bury them. The semiconductor light emitting element 802 is mounted on the lead 806, and connected to the lead 805 by a wire 808. The semiconductor light emitting element 802 is electrically fed through two leads 805, 806 to emit light, and the fluorescent element 810 absorbs the emitted light to release converted light. The semiconductor light emitting element 802 is a semiconductor that emits blue light, and the fluorescent material 810 is YAG:Ce fluorescent element that absorbs blue light from the light emitting element 802 and release yellow light.
With the light emitting device shown in FIG. 16, white light by mixture of two colors, namely the blue light from the semiconductor light emitting element 802 and the yellow light resulting from partial wavelength conversion by the fluorescent element 810, is extracted from a light release surface 812.
Through reviews, however, the Inventors have found that light emitting devices as shown in FIG. 16 involve the below-listed problems.
(1) The white balance largely fluctuates among light emitting devices.
(2) The white balance largely changes with the current value supplied.
(3) The white balance largely changes with the ambient temperature.
(4) The white balance largely changes with life of the semiconductor light emitting element 802.
All of those problems derive from essential characteristics of the blue light emitting element 802 used as the semiconductor light emitting element. That is, indium gallium nitride used as the light emitting layer of the blue light emitting element 802 is difficult to control strictly, and subject to fluctuation of emission wavelength among wafers on which it grows. In addition, it inherently varies largely in emission wavelength with the current supplied to the light emitting element 802 or with temperature. Furthermore, it exhibits a tendency of fluctuation of the emission wavelength while the supply of current and the emitting operation are continued.
Once the wavelength of blue light released from the blue light emitting element 802 fluctuates due to those reasons, its intensity gets out of balance with that of the yellow light from the fluorescent element 810, and their chromaticity coordinates will get out of order. It results in large changes of the white balance of the white light as the output of the device, and invites those problems, namely, fluctuation in brightness (luminance) and color (tone) of the white light obtained, bad reproducibility among products, and low mass productivity.
Moreover, the light emitting device shown in FIG. 16 inherently involves another problem that it is difficult to adjust the quantity of the fluorescent element in the resin enclosing the semiconductor element in accordance with the luminance of the light emitting element. Especially, emission from YAG:Ce with a high visible sensitivity is difficult to control because an error in quantity of the fluorescent element in the order of several micro grams (μg) influences the tone and the luminance.
Furthermore, this light emitting device is operative only in a narrow, limitative temperature range. If it is operated under, for example, 50° C. or higher temperature, the tone changes to bluish white. Such a temperature-caused change of color occurs due to a difference in temperature characteristics between the semiconductor element and the fluorescent element, namely because degradation of emission efficiency of the fluorescent element under a high temperature is larger than that of the degradation of emission efficiency of the semiconductor.
In addition, in case of the light emitting device shown in FIG. 16, the resin 804 containing the fluorescent element 810 for yellow emission has a “yellow” tone in its OFF state. That is, since the part lit “white” in the ON state looks “yellow” in the OFF state, its “appearance” is not good.